1. Field of the Invention
This invention relates to a method and apparatus for repairing protectively lined reactor vessels; and, more particularly, to a method and apparatus for repairing such vessels without introducing unwanted electrical paths through the vessel.
2. Description of the Prior Art
Protectively lined reactor vessels are widely utilized in chemical and allied industries to store a variety of corrosive and electrolytic materials such as aqueous solutions of acids, bases, and salts. These vessels typically comprise a conductive metal support wall, such as a steel shell, and an insulating protective inner lining such as glass, enamel, plastic, rubber, or the like. The metal wall provides the structural support required to hold the corrosive materials while the inner lining provides the corrosion resistance needed to protect the support wall from these materials.
Because of the importance of quickly repairing any break in the protective lining of a reactor vessel, such vessels are typically continuously monitored by electrical break detecting systems. Quick repair is of considerable importance in order to prevent destruction, by corrosion, of the underlying support wall. If the break is detected early, it can be readily repaired by the insertion of a corrosion resistant tantalum repair button. If, however, the break remains unnoticed for sometime, the support material may be substantially destroyed and a complete renewal of the protective lining by the manufacturer or even replacement of the vessel may be required.
The monitoring of a new vessel is relatively simple, but the subsequent insertion of tantalum repair buttons complicates the monitoring task by introducing electrical paths through the vessel. In a new vessel, breaks in the lining can be monitored by inserting an electrode into an electrolyte contained in the vessel and applying a potential difference between the electrode and the outer metal shell. A break is readily detected by the current flow resulting from the creation by the break of an electrical path through the insulating lining. Such breaks are typically repaired by the insertion of a tantalum screw or button therethrough and into the metal support wall.
Monitoring for and detection of subsequent breaks, however, is more complicated because the tantalum screw or button provides an unwanted electrical path from the electrolyte to the metal wall in which it is embedded. Consequently, any increased current flow due to a subsequent break might well be submerged in a high background level.
Prior methods for monitoring vessels thus repaired are less than completely satisfactory. One technique, disclosed in U.S. Pat. No. 3,555,414 issued to H. Deichelmann on Jan. 12, 1971, involves applying between the vessel wall and an electrode disposed in the electrolyte a direct current potential difference of such polarity that the wall is maintained at a positive potential with respect to the electrode. With such polarity and at appropriate magnitudes of voltage, the potential difference will effect a coating of the portion of the tantalum button exposed to the electrolyte with a thin passivating coating of tantalum oxide. This passivating coating, however, is readily subject to scratches and breakdown due to its very small thickness and, accordingly, does not permit reliable monitoring and detection.
An alternative technique disclosed in U.S. Pat. Nos. 3,831,085 and 3,858,114 issued to A. J. Kravatil and S. Voellmin, respectively, utilizes the natural "metal-to-metal" couple (battery effect) inherent with dissimilar metals disposed in electrolyte. In these techniques, an electrode of a metal other than steel, such as tantalum, is placed in the electrolyte. If a break in the lining should occur, a metal-to-metal couple is created between the steel and the metal electrode and a very sensitive galvanometer is used to detect the resulting current.
There are a number of difficulties with this alternative technique. First, the currents and voltages involved are very small and difficult to detect above background. Second, the technique is subject to spurious alarms because, in the normal use of such vessels it is common practice to insert into the electrolyte a variety of metal dip tubes which can activate the alarm system. Third, the frequent presence of the dissimilar metal dip tubes in the electrolyte contributes substantially to the deterioration of the tantalum plugs and the electrodes. This deterioration is the consequence of the generation of atomic hydrogen. Some of the atomic hydrogen permeates the tantalum, attacking it and causing embrittlement and cracking.